Draft:Space Power

Spacecraft depend on the Sun

Nearly all satellites use photovoltaics to generate the energy they need to run their instruments and stay alive. Solar power is an incredibly popular power choice for spacecraft, accounting for more than 90% of small and nanosats, as of 2021.

All the Apollo missions, mars rovers and deep space explorers have adopted alternative sources such as nuclear batteries . however these typically give relatively low levels of power for extremely long durations, using the steady heat generated to induce a voltage and enable current to flow through their power systems.

Utilising 'low-grade' heat fundamentally limits nuclear battery power and efficiency. Although nuclear reactors are a viable technology, the safety and responsibility aspects restrict the use of nuclear energy to spacecraft and missions which would not be able to succeed reasonably otherwise.

For these reasons, solar power will be an important technology for any spacecraft operating within the Martian orbit. The intensity of sunlight drops quite significantly this far away from the Sun, and it is recognised to be a problem even at this distance.

The trend toward smaller spacecraft, like CubeSats, exacerbate this problem, as they are too small to carry large solar panels. CubeSats must also accept that only the sides facing the Sun will harness energy – Depending on the mission parameters and orbital inclination, nearly half of their operational time is spent in shadow, exacerbating power limitations. Ultimately, this means they only operate for <5% of their total lifetime and at several millions of dollars each, it’s buying a Bugatti Veyron and only being able to take it out for a joyride – 30 minutes each day.

Limitations (OR Size Weight and Power)

The Sun

The Sun, although a fantastic source of energy, has it's limitations. The Sun won't get any brighter than 1.4 kW/m², at least at 1AU in our life-timescales, and sunlight can't overcome eclipse periods, or permanently shadowed regions as found in the Shackleton Crater, on the South pole of the Moon.

The easiest way to improve the power that is generated by the Sun is to increase the area of useful solar panels. Bigger satellites provide more area to mount photovoltaics, either as flat deployable arrays or even roll-out arrays. These aren’t necessarily practical for spacecraft as not only can the manufacturing costs increase drastically, being one of the most expensive components on the satellite, but it also influences the mass and associated launch cost increases - with a secondary effect on limiting the number of compatible rocket fairings, due to increased size and their consequences on scheduling flexibility.

Spacecraft are therefore designed for SWaP (Size, Weight and Power) - minimising Size and Weight, whilst maximising Power. As demand for more types of data increase (e.g. thermal imaging for ocean monitoring), satellites operators want to use more powerful instruments to deliver better quality data. Trying to push these limits can cause critical power failure – and total loss of control of your satellite.

Bigger panels also require stronger satellite structures, and panels are also affected in-flight by different forces such as Solar Radiation Pressure (SRP) and electrostatic forces. This will add additional costs to the satellite bus as well as the amount of propellant that is needed to maintain operations for the same expected lifetime.

Consequently, this presents a fundamental design problem - more power generation requires larger solar panels, increasing the size and weight more than is necessary for it's core purpose i.e. Earth Observation, RADAR or telecommunications. This can also present problems for new types space craft, such as in-orbit servicing spacecraft which demand high power payloads but need to be small and nimble, primarily welding robots which are critical for the assembly of Space Base Solar Power generators.

Solar Panels

Ironically, the nature of sunlight isn't ideal for solar panels. Not all the light from the sun is useful to all photovoltaics, as it depends on the specific bandgaps of the materials being used. Therefore, large parts of the solar spectrum are 'invisible' to the panel, and the lack of coherence of the light itself creates inefficiencies in the electricity conversion.

Increasing Spectrum Capture

It is possible to capture more of the spectrum on the same solar panel, without increasing the area, but instead by increasing the thickness. Multi-Junction cells utilise additional layers of complimentary substrates, sometime only 50 micrometers thick, to increase the useful spectrum. The different materials absorb different parts of the solar spectrum, dependent on their electron band-gaps, enabling more sunlight to be converted for a given area, improving the Power generated, whilst minimising the effect on Size and Weight.

However, there are limiting factors when using these as the addition of each layer escalates the risk of failure; a single malfunctioning layer can render the entire stack inoperative. Each layer demands more rare earth metals, typically III-V's. Although the amount of the actual material used in the solar array is quite small, the toxic waste can be about 2000 times more voluminous than the mined material.

When we consider the cumulative raw material requirements for all the photovoltaics of a CubeSat, or a constellation like Starlink with thousands of satellites, the environmental impact becomes significant. Factor in the mining methods, such as acid leaching and the generation of Naturally Occurring Radioactive Material (NORM), the sustainability of this approach should be considered and the number of materials required minimised as far as practicable.

Increasing Power Concentration

Lasers, with highly coherent beams and with their wavelength matched to the photovoltaic, can achieve much higher conversion efficiencies - up to 69% conversion rate compared to approx. 16% - 30% in sunlight . Their speciality in the conversion and matching has given them the name Laser Power Converters.

Lasers offer many benefits, enabling the light to be more concentrated, hence allowing a higher and faster charge than is available by the Sun, as well as providing light and power where the Sun cannot shine - i.e. Permanently Shadowed Regions of the Moon. This is an important milestone for space exploration as spacecraft have identified lots of frozen water in these shadows. This is an extremely valuable endeavour as we also suspect there is frozen hydrogen, ammonia, and interestingly for a space fusion reactor, Helium-3.

The Space Power Equation

Power Available = Time spent in Sun * Solar Panel Active Area *  % of Spectrum Visibility of Solar Panel * Solar Panel Conversion Efficiency

Time Spent in Sun

This is limited by the orbit LEO sun synchronous / GEO  are nearly 24/7. Other LEO orbits can spend up to 50% of the time in the shadow, similar to a Lunar mission where the night can last 2 weeks per month.

This factor can only be increased by changing the orbit of the spacecraft, or by using LASERs to provide light in the eclipse or night periods.

Solar Panel Active Area

This is the size of the solar panel which faces the Sun. As discussed in the Sun Section, there is limited opportunity to use large solar panels, either due to the size of the satellite itself, or even the external forces that will require a stronger chassis structure.

Spectrum Compatibility

This is affected by the placement of the solar panels with respect to atmospheric absorption, as well as the temperature of the source. For example, the Sun provides a black body radiation that approximates to 5250°C Blackbody Spectrum. The solar spectrum itself is a result of the natural processes found, and the radiation found at sea level is subject to atmospheric absorption, affecting the intensity of specific wavelengths, as a function of the gases of that planet's atmosphere.

LASERs offer a higher efficiency of conversion as they can be tuned specifically to the photovoltaics, ensuring the light will at least be 'visible' to the solar panel with the consequent conversion efficiency higher with engineered light, compared to the relatively stochastic nature of sunlight. This is supported by additional benefits such as the increased coherence of the light received, maximising the spectral compatibility to the panel and its associated efficiency, also minimising the temperature increase at the receiver, and consequently the overall conversion losses.